In order to minimize the vibration levels of, and assist in the diagnosis of faults in, the operation of, for example, helicopter rotors, it is desirable to be able to measure under various operating conditions the deflections of the rotor blades of the helicopter in directions which may be axial or tangential to the direction of rotation of the blade. In the case of helicopter rotor blades, the axial and tangential deflections are referred to as track and lag variations, respectively, and by measuring their values under various flight conditions it is possible to calculate and subsequently effect various adjustments to the rotor head in order to minimize overall vibration in the helicopter over a wide range of flight conditions. It is also desirable that such measurements should be made automatically, i.e., without operator involvement, and it should be possible to make these measurements both at night when the level of background light is very low and during the day when the background light levels can vary significantly depending on weather conditions, for example.
Systems employing parallax methods to measure blade track and lag variations and either using stroboscopic lights to illuminate the blade as it passes over a sensor or relying on the occlusion of ambient light by the rotating blade as it passes over the field of view of the sensor have been proposed in, for example, U.S. Pat. No. 2,960,908. This patent discloses a parallax interval sensing device which, as applied to the helicopter field, enables the distances of the individual blades from a sensor to be determined automatically. A tracker device is disclosed which utilizes an optical sensing device to determine the transit time of a helicopter blade between two fields of view with a fixed parallax angle therebetween. Given that the blade velocity is known, its distance from the sensing device is readily determinable. The sensing device has optics designed to define two fields of view inclined at a defined angle and a photoelectric cell responsive to interruption by a blade of the light paths in the fields of view to produce a pulsatory signal containing information as to the time taken for the leading (or trailing) edge of the blade to transit between the fields of view. The further the distance of the blade from the sensor, the longer will be this transit time, so the measure of transit time provides a measure of blade distance from the sensor.
The arrangement described in this document requires a high contrast between the blade and the background light in order to provide a clear and discernable signal which changes with time. Where the intensity of the light to be obscured by the blade decreases, then the change of signal is smaller and the accuracy of the measurement and the resolution of the measurement decreases as a result.
It is also possible to operate the system of U.S. Pat. No. 2,960,908, during the hours of darkness by illuminating the side of the rotor adjacent to and facing the device by artificial means. For example, the patentees suggest in the operators' manual of their device the use of a 600W landing light GE4559 or equivalent. To make use of such a light, the system is capable of operating with light (illuminated) blades on a dark background, as well as with dark blades against a light background during normal daytime use.
In general switching over between the two modes of operation, i.e., during the day relying on background light or at night relying on artificial illumination, may be either manual, or preferably, automatic. Despite the inherent difference between optical signal employed, i.e., dark against light or light against dark, the calculation of blade height is not altered.
The proposed method of operating the apparatus of U.S. Pat. No. 2,960,908 at night has a number of disadvantages, which stem from the low level of illumination of the blade which is achievable in practice. Firstly, an intense light source consuming considerable electrical power must be used. The intensity of the light must not fluctuate significantly during the course of measurement, and it is generally convenient to operate the light from a DC supply, for example the 28V DC supply provided on board the aircraft. However, the power which may be drawn from this supply is in general quite limited, and this sets a limit on the maximum achievable light intensity incident upon the rotor blade when the system is used at night.
Another disadvantage is that rotor blades, particularly those fitted to military helicopters, are frequently painted in a dark or black anti-reflection finish in order to reduce the possibility of detecting the aircraft under combat conditions, and this further reduces the level of light which the detector must be capable of detecting.